package dns01 import ( "errors" "fmt" "net" "strings" "sync" "time" "github.com/miekg/dns" ) const defaultResolvConf = "/etc/resolv.conf" // dnsTimeout is used to override the default DNS timeout of 10 seconds. var dnsTimeout = 10 * time.Second var ( fqdnSoaCache = map[string]*soaCacheEntry{} muFqdnSoaCache sync.Mutex ) var defaultNameservers = []string{ "google-public-dns-a.google.com:53", "google-public-dns-b.google.com:53", } // recursiveNameservers are used to pre-check DNS propagation. var recursiveNameservers = getNameservers(defaultResolvConf, defaultNameservers) // soaCacheEntry holds a cached SOA record (only selected fields). type soaCacheEntry struct { zone string // zone apex (a domain name) primaryNs string // primary nameserver for the zone apex expires time.Time // time when this cache entry should be evicted } func newSoaCacheEntry(soa *dns.SOA) *soaCacheEntry { return &soaCacheEntry{ zone: soa.Hdr.Name, primaryNs: soa.Ns, expires: time.Now().Add(time.Duration(soa.Refresh) * time.Second), } } // isExpired checks whether a cache entry should be considered expired. func (cache *soaCacheEntry) isExpired() bool { return time.Now().After(cache.expires) } // ClearFqdnCache clears the cache of fqdn to zone mappings. Primarily used in testing. func ClearFqdnCache() { muFqdnSoaCache.Lock() fqdnSoaCache = map[string]*soaCacheEntry{} muFqdnSoaCache.Unlock() } func AddDNSTimeout(timeout time.Duration) ChallengeOption { return func(_ *Challenge) error { dnsTimeout = timeout return nil } } func AddRecursiveNameservers(nameservers []string) ChallengeOption { return func(_ *Challenge) error { recursiveNameservers = ParseNameservers(nameservers) return nil } } // getNameservers attempts to get systems nameservers before falling back to the defaults. func getNameservers(path string, defaults []string) []string { config, err := dns.ClientConfigFromFile(path) if err != nil || len(config.Servers) == 0 { return defaults } return ParseNameservers(config.Servers) } func ParseNameservers(servers []string) []string { var resolvers []string for _, resolver := range servers { // ensure all servers have a port number if _, _, err := net.SplitHostPort(resolver); err != nil { resolvers = append(resolvers, net.JoinHostPort(resolver, "53")) } else { resolvers = append(resolvers, resolver) } } return resolvers } // lookupNameservers returns the authoritative nameservers for the given fqdn. func lookupNameservers(fqdn string) ([]string, error) { var authoritativeNss []string zone, err := FindZoneByFqdn(fqdn) if err != nil { return nil, fmt.Errorf("could not determine the zone: %w", err) } r, err := dnsQuery(zone, dns.TypeNS, recursiveNameservers, true) if err != nil { return nil, err } for _, rr := range r.Answer { if ns, ok := rr.(*dns.NS); ok { authoritativeNss = append(authoritativeNss, strings.ToLower(ns.Ns)) } } if len(authoritativeNss) > 0 { return authoritativeNss, nil } return nil, errors.New("could not determine authoritative nameservers") } // FindPrimaryNsByFqdn determines the primary nameserver of the zone apex for the given fqdn // by recursing up the domain labels until the nameserver returns a SOA record in the answer section. func FindPrimaryNsByFqdn(fqdn string) (string, error) { return FindPrimaryNsByFqdnCustom(fqdn, recursiveNameservers) } // FindPrimaryNsByFqdnCustom determines the primary nameserver of the zone apex for the given fqdn // by recursing up the domain labels until the nameserver returns a SOA record in the answer section. func FindPrimaryNsByFqdnCustom(fqdn string, nameservers []string) (string, error) { soa, err := lookupSoaByFqdn(fqdn, nameservers) if err != nil { return "", err } return soa.primaryNs, nil } // FindZoneByFqdn determines the zone apex for the given fqdn // by recursing up the domain labels until the nameserver returns a SOA record in the answer section. func FindZoneByFqdn(fqdn string) (string, error) { return FindZoneByFqdnCustom(fqdn, recursiveNameservers) } // FindZoneByFqdnCustom determines the zone apex for the given fqdn // by recursing up the domain labels until the nameserver returns a SOA record in the answer section. func FindZoneByFqdnCustom(fqdn string, nameservers []string) (string, error) { soa, err := lookupSoaByFqdn(fqdn, nameservers) if err != nil { return "", err } return soa.zone, nil } func lookupSoaByFqdn(fqdn string, nameservers []string) (*soaCacheEntry, error) { muFqdnSoaCache.Lock() defer muFqdnSoaCache.Unlock() // Do we have it cached and is it still fresh? if ent := fqdnSoaCache[fqdn]; ent != nil && !ent.isExpired() { return ent, nil } ent, err := fetchSoaByFqdn(fqdn, nameservers) if err != nil { return nil, err } fqdnSoaCache[fqdn] = ent return ent, nil } func fetchSoaByFqdn(fqdn string, nameservers []string) (*soaCacheEntry, error) { var err error var in *dns.Msg labelIndexes := dns.Split(fqdn) for _, index := range labelIndexes { domain := fqdn[index:] in, err = dnsQuery(domain, dns.TypeSOA, nameservers, true) if err != nil { continue } if in == nil { continue } switch in.Rcode { case dns.RcodeSuccess: // Check if we got a SOA RR in the answer section if len(in.Answer) == 0 { continue } // CNAME records cannot/should not exist at the root of a zone. // So we skip a domain when a CNAME is found. if dnsMsgContainsCNAME(in) { continue } for _, ans := range in.Answer { if soa, ok := ans.(*dns.SOA); ok { return newSoaCacheEntry(soa), nil } } case dns.RcodeNameError: // NXDOMAIN default: // Any response code other than NOERROR and NXDOMAIN is treated as error return nil, fmt.Errorf("unexpected response code '%s' for %s", dns.RcodeToString[in.Rcode], domain) } } return nil, fmt.Errorf("could not find the start of authority for %s%s", fqdn, formatDNSError(in, err)) } // dnsMsgContainsCNAME checks for a CNAME answer in msg. func dnsMsgContainsCNAME(msg *dns.Msg) bool { for _, ans := range msg.Answer { if _, ok := ans.(*dns.CNAME); ok { return true } } return false } func dnsQuery(fqdn string, rtype uint16, nameservers []string, recursive bool) (*dns.Msg, error) { m := createDNSMsg(fqdn, rtype, recursive) var in *dns.Msg var err error for _, ns := range nameservers { in, err = sendDNSQuery(m, ns) if err == nil && len(in.Answer) > 0 { break } } return in, err } func createDNSMsg(fqdn string, rtype uint16, recursive bool) *dns.Msg { m := new(dns.Msg) m.SetQuestion(fqdn, rtype) m.SetEdns0(4096, false) if !recursive { m.RecursionDesired = false } return m } func sendDNSQuery(m *dns.Msg, ns string) (*dns.Msg, error) { udp := &dns.Client{Net: "udp", Timeout: dnsTimeout} in, _, err := udp.Exchange(m, ns) if in != nil && in.Truncated { tcp := &dns.Client{Net: "tcp", Timeout: dnsTimeout} // If the TCP request succeeds, the err will reset to nil in, _, err = tcp.Exchange(m, ns) } return in, err } func formatDNSError(msg *dns.Msg, err error) string { var parts []string if msg != nil { parts = append(parts, dns.RcodeToString[msg.Rcode]) } if err != nil { parts = append(parts, fmt.Sprintf("%v", err)) } if len(parts) > 0 { return ": " + strings.Join(parts, " ") } return "" }